Low-Temperature Cavity-Enhanced Spectroscopy with a Diode Laser: Vibration–Rotation Absorption of CO (3ν) at 80 K and CH₃C≡C–H (2ν₁) at 165 K

Abstract

A rotational resolution cavity-enhanced absorption spectroscopy (CEAS) technique is presented for infrared (IR) absorption studies at temperatures between 77 and 300 K. The cavity consists of an aluminum tube with two highly reflecting near-IR mirrors at each end. The cavity is attached to the cold head of a liquid nitrogen cryostat inside a vacuum chamber. In this way, all the cavity components are the temperature of the cold head. The spectra are obtained with a tunable diode laser and lock-in detection. The laser is modulated with an electro-optic modulator and coupled to the optical cavity. To illustrate the use of the technique, the vibrational–rotational overtone transitions (υ = 0→3) of carbon monoxide at 295 and 80 K and the acetylenic C–H (υ = 0→2) bond of CH3C≡C–H at 293 and 165 K are obtained. Temperatures are obtained with a Boltzmann distribution calculation that involves rotational intensities and energies from the spectra. Spectroscopic constants agree with previous determinations using other methods. An average of signals over the entire range of the absorption produces a rotational intensity distribution with some deviations. The calculated temperature based on the Boltzmann distribution is in very good agreement with the temperature measured with a silicon temperature sensor attached to the cell. Comparison with other experimental approaches to CEAS and the importance of the technique for applications in astrochemistry are discussed.